Influence of atomic-scale defects on coherent phonon excitations by THz near fields in an STM

TL;DR

This study uses terahertz scanning tunneling microscopy to investigate how atomic-scale defects in MoTe2 influence the excitation of coherent phonons, revealing defect-dependent modulation of vibrational modes.

Contribution

It demonstrates defect-dependent control of coherent phonon excitation in MoTe2 using THz-STM, highlighting nanoscale defect influence on vibrational mode coupling.

Findings

Long-lived out-of-plane and in-plane phonon modes observed.

Mode excitation strength varies near defects due to local band bending.

Defect-tunable coupling enables nanoscale control of vibrational modes.

Abstract

Coherent phonons describe the collective, ultrafast motion of atoms and play a central role in light-induced structural dynamics. Here, we employ terahertz scanning tunneling microscopy (THz-STM) to excite and detect coherent phonons in semiconducting 2H-$MoTe_{2}$ and resolve how their excitation is influenced by atomic-scale defects. In a THz pump-probe scheme, we observe long-lived oscillatory signals that we assign to out-of-plane breathing and in-plane shear modes, which are both forbidden in the bulk. Remarkably, the relative excitation strength of these modes varies near defects, indicating that local band bending modulates the coupling to the THz field. This defect-tunable coupling offers new opportunities to control material properties via selective excitation of vibrational modes at the nanoscale.